1. Field of the Invention
This invention is concerned with a novel sorbent. In particular, it is concerned with a sorbent comprising sulfur occluded in naturally occurring manganese nodules. The novel sorbent is highly effective for removing and/or recovering certain trace metals as more particularly described hereinafter, which metals may be undesirable contaminants in fluids. The sorbent is particularly effective for removing mercury contaminant in a gas stream.
2. Prior Art
Undesirable metal contaminants, more fully described below, are found with reasonable frequency in fluids. The term fluids, as used herein, refers to liquids and gases such as water, gasoline, and other organic liquids, air, natural gas, hydrogen and the like. The metal contaminant often is undesired because of its toxicity and the likelihood that human or animal exposure to it will occur. In other instances, the metal contaminant is undesired because it adversely affects the performance or behavior of the fluid for a particular use. In still other instances toxicity together with other adverse effects dictate removal of the contaminant or at least reduction in the amount present in the fluid.
In the transportation and storage of lead-free gasoline it sometimes happens that lead contamination occurs due to the presence of residues of leaded gasoline in the storage tank, for example. The contaminant will adversely affect the catalytic converter of an automobile and the gasoline thus becomes unsuited for its intended use. U.S. Pat. No. 3,856,664 to Whitehurst describes a method for removing such contaminant by contacting the gasoline with a sorbent consisting of cross-linked polystyrene reacted with sulfur.
Another contaminant which occurs in different environments is mercury. This metal is highly toxic. The "Toxic Substances List" published by the U.S. Department of Health, Education and Welfare, N10SH 76-191 (1976 Ed.) establishes an allowable concentration in air of 0.05 mg/m.sup.3 (0.05 milligrams of mercury per cubic meter of air). The vapor pressure of metallic mercury at 25.degree. C. corresponds to a concentration of about 20 mg/m.sup.3 in saturated air, which is about four hundred times the safe level.
Gases produced during the smelting of zinc contain toxic levels of mercury.
Certain natural gas fields produce gas that contains significant levels of mercury. The liquified gas from these fields corrodes the aluminum fittings which it contacts.
The chlor-alkali process for the manufacture of chlorine and caustic soda by the electrolysis of brine with the use of a mercury anode presents substantial mercury emissions problems. On decomposition of the amalgam, the by-product hydrogen that is formed is saturated with mercury vapor at elevated temperature and in addition the hydrogen carries with it extremely small particles of the metal itself. Also, the ventillation air stream for the so-called cell end-box and the cell-room ventillation air contain mercury.
Processes for removing mercury contaminant from gas streams have been proposed or are in use. Tubular-type mist eliminators, for example, remove the larger entrained particles of metal, but of course have no effect on the metal vapor. Processes effective for removal of vapor may be preceded by cooling and demisting. Sulfur- or iodine-impregnated activated carbon has been proposed as a sorbent for mercury vapor. See, for example, U.S. Pat. No. 3,194,629 to Dreibelbis et al. U.S. Pat. No. 4,101,631 to Ambrosini et al. Describes a process for removing mercury vapor from gas streams by sorption on a crystalline zeolitic molecular sieve having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio from 2 to 20 and loaded with at least 0.5 weight percent elemental sulfur. The above-described sorbents are presumed effective for an extended period of time, i.e. months to years, but are not regenerable. Furthermore, these sorbents are synthetic and relatively costly.
A cyclical process known as the Purasiv Hg adsorption process utilizes a proprietary crystalline aluminosilicate molecular sieve and operates on a short cycle (about 24 hours for sorption) in which it is regenerated at high temperature. This process has been studied in detail in connection with its application in chlor-alkali plants, and the results published in a report entitled "Molecular Sieve Mercury Control Process in Chlor-Alkali Plants", by M.Y. Anastas, U.S. Environmental Protection Agency Report PB 251-203, January 1976, the entire contents of which are incorporated herein by reference.
U.S. Pat. No. 4,093,541 to Piccinini et al. discloses a method for removing metallic mercury from industrial sewage water by adsorption on activated carbon.
Manganese nodules is the descriptive term for a naturally occurring underwater deposit of a rock-like solid. These deposits, found on the floor of bodies of water, contain a high content of manganese mineral associated with other metals including iron, cobalt, nickel, and copper. They are found in abundance in the Atlantic and Pacific Oceans and in Lake Michigan. The nodules are characterized by a large surface area, i.e. in excess of about 100 square meters per gram. They have a wide variety of shapes but most often those from the oceans look like potatoes, while those from bodies of fresh water, such as Lake Michigan, tend to be smaller in size. Their color varies from earthy black to brown depending upon their relative manganese and iron content. The nodules are porous and light, having an average specific gravity of about 2.4. Generally, they range from 1/8 inch to 9 inches in diameter but may extend up to considerably larger sizes approximating 4 feet in length and 3 feet in diameter and weighing as much as 1,700 pounds. In addition to the metals mentioned above, the nodules contain silicon, aluminum, calcium and magnesium, and small amounts of molybdenum, zinc, lead, vanadium, and rare earth metals.
The manganese nodules substantially as mined, or recovered, from the floor of the body of the water in which they occur, may be used to prepare the sorbent of this invention. Thus, the nodules, as mined, possibly after washing to remove salt and mud or other loose material from the surface of the nodules, may be crushed and sized to obtain a desired particle size.
It is an object of this invention to provide a novel sorbent for removing a metal contaminant from a fluid. It is a further object of this invention to provide a process for removing lead from liquid hydrocarbons. It is a further object of this invention to provide a process for removing mercury contamination from gas streams, particularly from air or from hydrogen. It is a further object of this invention to provide a novel process for recovering valuable metals. Other objects will become apparent to one skilled in the art on reading this entire specification including the claims thereof.